Large airplanes often have an on-board auxiliary power unit to provide electrical power and compressed air to various systems throughout the airplane. When the airplane is on the ground, the auxiliary power unit is the main source of power to drive the environmental control systems, air driven hydraulic pumps, and the starters for the engines. Auxiliary power units are generally used while the airplane is on the ground, but they may also provide pneumatic and electric power during flight.
Auxiliary power units are lubricated with oil that is cooled by an oil cooler. An active fan is typically used to push air across the oil cooler and through the compartment in which the auxiliary power unit is positioned. Active fans are driven at high speeds by a shaft and complex gear assembly that is operatively connected to the auxiliary power unit. In light of the high operating speeds and numerous, complex mechanical components, active fans may fail over extended periods of use. Active cooling systems Failures have a negative impact on the reliability of auxiliary power units which increase the cost of operating the airplane and raises safety concerns. Accordingly, it would be desirable to provide a cooling system that reduces or eliminates the number of complex, moving mechanical pans.